U.S. Pat. No. 4,824,352, discloses a pellet mill for processing coarse and long fibers, especially straw, into animal feed. The material is fed by a screw conveyor into an expanding hopper, which is arranged in the intake wedge of two hollow rolls, which are driven in a mechanically controlled manner and engage each other in the manner of toothed gears. A plurality of holes tapering in the radial direction lead from the root of the tooth into the interior of the hollow rolls. The teeth are elongated in the axial direction and roll on each other. The straw material introduced into the wedge area of the hollow rolls is compacted by the teeth penetrating into the gashes and it is pressed through the radial holes into strands, which break off in the hollow space of the hollow rolls and are removed axially from the hollow spaces.
Practical work with such pelletizing devices shows that trouble-free pressing of large amounts of straw material is problematic, because the straw material coils up around the teeth of the hollow bodies and accumulates in the root of the teeth by the radial holes, increasing resistance, so that the rotating bodies keep becoming blocked. Such devices are definitely unsuitable for preparing pressed objects of high density from straw material for use as a fuel material or for industrial purposes, because the frictional resistance of the meshing teeth as well as of the conical holes is too high, so that the energy needed to drive the hollow rolls increases excessively and cannot be ensured with the conventional means at all. In addition, the residues of straw material deposited on the hollow roll are heated up to pyrolysis temperatures due to the increased compression action, and the carbonized residues thus formed also lead to breakdown of the pelletizing device in a short time.